Foaming is a common problem in aerobic, submerged fermentations. Foaming is caused by the sparging of gas into the fermentation medium for the purpose of providing oxygen for the growth of the aerobic organism being cultivated (e.g. bacteria, yeasts, fungi, algae, cell cultures). If the fermentation medium contains surface active components such as proteins, polysaccharides or fatty acids, then foam can be formed on the surface of the medium as the sparged gas bubbles disengage from the liquid. Foaming creates a number of problems including the undesirable stripping of product, nutrients, and cells into the foam, and can make process containment difficult. A known method for controlling foaming is to use antifoams, of which several types are commonly used: silicone-based (e.g. polydimethylsiloxanes), polyalkylene glycols (e.g. polypropylene glycol), fatty acids, polyesters and natural oils (e.g. linseed oil, soybean oil). Antifoams replace foam-forming components on bubble surfaces, resulting in destruction of the foam by bubble coalescence. Antifoams are added at the start of and/or during the fermentation.
When the fermentation product is intended for use in foods, personal products or medicine, it is highly desirable that the product is excreted by the producing organism into the fermentation medium (i.e. extra-cellular, rather than intra-cellular production). This avoids the need to disrupt the cells by physical or chemical means in order to release the product for recovery. By maintaining the cells intact, the cellular material can be easily separated from the product so that it is free of intracellular and genetic material which is usually regarded as an undesirable contaminant. This can be especially important when the producing organism has been genetically modified. However, extra-cellular production may intensify the degree of foaming in the fermenter, especially if the product facilitates foam formation or enhances foam stability, for example a biosurfactant or a hydrophobin. The use of antifoams presents a particular problem in the extra-cellular production of such foaming agents for two reasons: firstly the amount of antifoam required is increased because the foaming agent itself contributes to foaming in the fermenter. Secondly, it is not necessary to remove the antifoam from most fermentation products since it is present in low concentrations which do not affect the functionality of the product. However, when the fermentation product is a foaming agent, the antifoam must be substantially removed since the presence of antifoam in the product will impair its functionality.
Bailey et al, Appl. Microbiol. Biotechnol. 58 (2002) pp 721-727 disclose the production of hydrophobins HFB I and HFB II by the fermentation of transformants of Trichoderma reesei. An antifoam (Struktol J633) was used to prevent foaming and the hydrophobin was purified using aqueous two phase extraction. However separation methods such as aqueous two phase extraction or chromatographic processes are expensive and may require food-incompatible chemicals.
It has now been found that, rather than removing the antifoam from the solution it is possible to remove the hydrophobin.